This invention relates to tensioning devices and more particularly to devices for applying a carefully controlled tension to members such as stretcher bars upon which an artist's canvas is mounted.
In mounting a canvas on a supporting frame, it is common practice to form a frame from four stretcher bars, each of which is provided with mitered tongue and groove corners which mate with corresponding corners of adjacent bars. Generally, upon affixing the canvas to such stretcher bars, such as by stapling or tacking the canvas edges to the bars, the canvas is rendered taut by the application of an outwardly directed tension force against the stretcher bar frame. Commonly, such tension is applied by driving wooden wedges, for example by a hammer or the like, into the open corner portion of mated bars. This technique for tensioning stretcher bars has the disadvantage of providing no means for retaining wedges in place and, over a period of time shrinkage of both wedges and stretcher bars will result in wedges falling from a frame with a concommitant loss of canvas tension. A further disadvantage of wedge tensioning techniques has become apparent during the cleaning and reframing of older canvases. It has been found that when tension is applied to a reframed, older canvas, a great deal of care must be exercised in the process so as not to apply excessive amounts of tension which can easily result in the production of cracks or other faults in valuable canvases. However, in utilizing a wedge tensioning technique as aforesaid, it is extremely difficult to control the intensity of force applied to a wedge by a hammer or the like. Accordingly, by striking a wedge with too great an impact, cracks are easily produced in older canvases during reframing. Thus, a clear need exists for devices capable of applying a carefully controlled tension force to stretcher bars and assuring that such tension is securely maintained.
In order to avoid the foregoing adverse effects encountered upon employing wedge techniques for tensioning stretcher bars, it has been proposed to utilize set screws operating against stretcher bars as illustrated in U.S. Pat. No. 1,562,153. The device described therein includes a single corner bracket affixed to both stretcher bars by means of nails or the like with set screws threadably engaged with each bracket arm and effective to apply a tension force in a direction parallel to the longitudinal axis of such nails. Adjustment of set screws however, can result in the separation of bracket arms from the stretcher bar to which such bracket is attached. In U.S. Pat. No. 2,491,600, a tensioning device for stretcher bars includes a wedge member formed integrally with an angle portion and upon adjustment of set screws, a force is applied against the stretcher bars. This device however, suffers from the disadvantage of other wedge type tensioning devices, namely upon aging, the wedge may slip within slots formed in the stretcher bars with a corresponding loss of canvas tension.